1. Field of the Invention
The present invention relates to an imaging lens system and particularly, to an imaging lens system suitable for use in an image-taking device using an image sensor element such as CCD, CMOS and the like.
2. Description of the Related Art
In recent years, multi-media have been developed remarkably. For example, the demand for a camera utilizing a solid image sensor element such as CCD, CMOS and the like mounted in a mobile phone or the like, e.g., a CCD camera has been increased. Such a CCD camera is required to be mounted in a limited space and hence, it is desired that the CCD camera is small-sized and lightweight. Therefore, it is required that an imaging lens system used in the CCD camera is also small-sized and lightweight.
Various optical systems for such an imaging lens system have been conventionally proposed depending on the application of the imaging lens system, and optical systems intended to be small-sized and lightweight have been also proposed for cameras to be mounted in a mobile phone as described above.
For the above-described camera utilizing the solid image sensor, the requirements for not only reductions in size and weight but also an increase in resolution have been more and more increased recently.
However, to realize the reductions in size and weight, a restriction is naturally imposed on even the entire length (a dimension in a direction of an optical axis) of an optical system incorporated into a camera and for this reason, a lens arrangement is obliged to be simple. When a lens system of a single-lens arrangement which is simplest is yet used, the following problem is encountered: The entire length of the optical system can be fallen into a satisfactory range, but an astigmatism and a chromatic aberration cannot be corrected satisfactorily, and a sufficient resolution (e.g., 640xc3x97480 dots) cannot be obtained.
An optical system using a lens system of a two-lens arrangement has been also proposed. Even in this case, however, the restriction on the entire length of the optical system and the required resolution are not satisfied simultaneously and insufficient.
On the other hand, if the lens system is formed in a three-lens arrangement, the required resolution can be satisfied, but the restriction on the entire length of the optical system cannot be satisfied.
The fact is that an optical system sufficient and effective for solution of the subject that a sufficient high resolution is obtained while reducing the entire length of the optical system has been still proposed.
Accordingly, it is an object of the present invention to provide an imaging lens system, wherein a sufficient high resolution can be obtained, while meeting the needs for reduction in entire length of the optical system to provide the compactness.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided an imaging lens system comprising, in the named order from the side of an object toward an image surface, a light amount diaphragm, a first lens having a positive power with a main power on the side closer to the image surface, a meniscus-shaped second lens having a negative power with a strong concave surface turned toward the image surface.
With the arrangement of the first feature, various aberrations can be corrected satisfactorily to provide a sufficient high resolution. Despite use of the two lenses, the entire length of the lens system can be reduced sufficiently and thus, it is possible to realize the imaging lens system which is compact and has a high resolution and a high quality.
According to a second aspect and feature of the present invention, in addition to the first feature, the following condition expressions (1) and (2) are satisfied:
xe2x88x921.9 less than f/f2  less than xe2x88x920.5xe2x80x83xe2x80x83(1) 
1.3 less than f/f1 less than 2.1xe2x80x83xe2x80x83(2) 
wherein
f is a focal length of a combination of the lenses;
f2 is a focal length of the second lens; and
f1 is a focal length of the first lens.
With the arrangement of the second feature, the entire length can be reduced, while ensuring a sufficient back focus, and the resolution and the quality of an image can be further enhanced, by ensuring that the second lens satisfies the expression (1) and the first lens satisfies the expression (2).
According to a third aspect and feature of the present invention, in addition to the first or second feature, the following condition expressions (3) and (4) are satisfied:
xcexd1 greater than 50xe2x80x83xe2x80x83(3) 
xcexd2 less than 40xe2x80x83xe2x80x83(4) 
wherein
xcexd1 is an Abbe number of the first lens, and
xcexd2 is an Abbe number of the second lens.
With the arrangement of the third feature, the generation of a chromatic aberration in the first lens can be reduced by ensuring that the first lens satisfies the condition expression (3), and a chromatic aberration, still remaining even when the expression (3) is satisfied, can be corrected further positively in the second lens by ensuring that the second lens satisfies the condition expression (4). Thus, it is possible to realize the imaging lens system in which the chromatic aberration is further corrected.
According to a fourth aspect and feature of the present invention, in addition to the second or third feature, the following condition expressions are satisfied:
0.3 f less than d1xe2x80x83xe2x80x83(5) 
d2 less than 0.3 fxe2x80x83xe2x80x83(6) 
wherein
d1 is a thickness of the first lens at its center, and
d2 is a thickness of the second lens at its center.
With the arrangement of the fourth feature, the astigmatism and the distortion can be corrected satisfactorily to further enhance the resolution and the quality of the image, and a sufficient back focus required for the structure can be obtained.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.